1. Field of the Invention
This invention relates to an image read-out apparatus wherein a recording medium, on which an image has been recorded, is two-dimensionally scanned with a light beam, and light which is radiated out of the recording medium during the scanning and which represents the image is detected with a photoelectric sensor, a time-serial image signal being thereby generated.
1. Description of the Prior Art
Techniques for reading out an image, which has been recorded on a recording medium, in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields.
For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on photographic film, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In the image recording and reproducing systems described above, the intensity of light, which is radiated out of a recording medium (such as X-ray film or a stimulable phosphor sheet) carrying an image recorded thereon and which represents the image, may detected with a detection capacity of approximately 3 orders of ten (i.e. such that the highest intensity of the light is detected to be approximately 1.0.times.10.sup.3 when the lowest intensity of the light is detected to be 1.0). In such cases, theoretically, a visible image can be reproduced from the image signal thus obtained, which visible image contains broad image information ranging from a low image density region to a high image density region.
However, the mean intensity of light representing an image is not necessarily constant and is generally unknown before the image read-out operation is carried out. Therefore, if the dynamic range of a photoelectric sensor, which detects the light representing the image, or the dynamic range of the system, which processes the image signal, is as narrow as 3 orders of ten, the low image density region or the high image density region of the information representing the image, which is recorded on the recording medium, cannot be detected normally. This problem occurs unless the mean intensity of the light representing the image matches with the gain or the signal level in the photoelectric sensor or the signal processing system. When a visible image is reproduced from the image signal thus obtained, part of the information about the original image is lost in the visible image.
In order to eliminate the aforesaid problems, the method described below has heretofore been employed. Specifically, such that no problem may occur when the mean intensity of light representing an image fluctuates slightly, a high-performance photoelectric sensor is used which has a wide dynamic range and can accurately detect the intensity of light over a range of, for example, at least 4 orders of ten. Also, a signal processing system is constituted of high-performance circuit devices, which can process a signal over a dynamic range of at least 4 orders of ten, and the configuration of the circuits is devised in a specific manner. However, with this method, because the high-performance photoelectric sensor and the high-performance circuit devices must be used, the manufacturing cost cannot be kept low. Also, a wide dynamic range cannot be achieved unless the speed, with which a signal is processed, is kept low.
As another method for eliminating the aforesaid problems, the method described below has also been proposed. Specifically, in cases where an image, which has been stored on, for example, a stimulable phosphor sheet, is read out, a preliminary read-out operation is carried out before a final read-out operation, wherein the image is read out at a sufficiently high accuracy, is carried out. With the preliminary read-out operation, the stimulable phosphor sheet is exposed to comparatively weak stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation. During the detection of the emitted light, the gains in a photoelectric sensor and a signal processing system are set to low values. (If the gains are set to low values, the resolution of image density will deteriorate.) A preliminary read-out image signal, which has been obtained from the preliminary read-out operation, is then analyzed. From the results of analysis, the level of light, which will be emitted by the stimulable phosphor sheet during the final read-out operation, is predicted. During the final read-out operation, the gains in the photoelectric sensor and the signal processing system are adjusted to appropriate values in accordance with the results of prediction. In this manner, an image signal is obtained which accurately represents the image stored on the stimulable phosphor sheet. However, with the proposed method, in order to generate a single image signal, two read-out operations must be carried out on the stimulable phosphor sheet. Therefore, the processing capacity of the image read-out apparatus per unit time cannot be kept large.